Why use 3S instead of 2S with SBEC

Assuming the guy at (listen to the podcast!)
http://www.radicalrc.com/blog/?p=82
is correct: (and assuming I wrote it down correctly)
((Ah * vBatt) / vReg) * (SBEC efficiency% / 100) = Ah available after
regulation
where

Ah

amp hours of the battery (mAh / 1000)

vBatt

nominal voltage of the battery

vReg

output voltage of the regulator

SBEC efficiency

percent efficiency of the SBEC (about 85% for a good one like Castle Creations)

SBEC

SWITCHING BEC, not a linear one, which just gets hot with the excess power)

so, 1350mAh 2S (regulating to 5 volts)
(1.35 * 7.4) / 5 = 1.998 * 0.85 = 1.6983 Ah
1000mAh 3S
(1 * 11.1) / 5 = 2.22 * 0.85 = 1.887 Ah
so according to that calculation the 3S will last slightly longer.
All theoretical of course, and at this sort of difference you're
probably better off with whichever is the lighter setup and just charge
between flights or swap packs, whatever suites you. Not forgetting that
you don't want to use more than 70% to 80% of the available battery
capacity anyway, else you risk damaging the pack.
just for fun, what about a 1350mAh 3S pack?
(1.35*11.1) / 5 = 2.997 * 0.85 = 2.54745 Ah
wow! that is a big step up over the 2S